Field of the Invention
The present invention relates to an image processing apparatus and method, and an image capturing apparatus, and more particularly to an image processing apparatus and method, and an image capturing apparatus for performing tone conversion of image signals.
Description of the Related Art
Conventionally, an output dynamic range of an output device such as a television or a display is not sufficient, and images can be displayed only in a dynamic range which is considerably narrower than a dynamic range of an actual subject. For this reason, when a subject is shot with the image capturing apparatus, it is necessary to compress a wide dynamic range of the video signal into the dynamic range of display of the output device. When such processing was performed, a video was displayed on the output device in a state different from the actual appearance of the subject, and the realistic feeling was lost.
However, due to recent technological innovation, the maximum luminance output of an output device has greatly improved and the dynamic range that can express the tone of an image is widened, which makes it possible for an output device to output an image in a dynamic range that can cover most of the human visual characteristics. Along with such the improvement in the dynamic range of an output device, the conversion characteristic of the output device for displaying an image with an extended dynamic range is standardized as SMPTE Standard ST 2084:2014, as reported in Report ITU-R BT.2246-1 (08/2012)/The present state of ultra high definition television.
As shown in the above report, it has been scientifically verified that human visual characteristics are such that recognizable JND (Just Noticeable Difference) differs depending upon perceived luminance. Based on this fact, SMPTE Standard ST 2084:2014 associates the code value of the image signal with the luminance value displayed by the output device, standardizes them. For this reason, it is expected that the image signal input to the output device is photoelectrically converted based on the inverse function of this conversion characteristic.
On the other hand, a case may be assumed in which an image may be later displayed both on a display device having a wide dynamic range and on a display device having a conventional dynamic range. In such a case, rather than performing tone conversion conforming to the respective standards on an image at the time of shooting, the recorded image becomes more versatile and its value as content also improves by performing coding that can correspond to both of the wide dynamic range and the conventional dynamic range on the image at the time of shooting, and then performing tone conversion that conforms to the respective standards on the image at the time of post-editing after recording. At that time, it is necessary to assume that the image is converted into the image with a wide dynamic range at the time of post-editing, and an image coding method that efficiently saves subject information of the real world as an image signal is required.
Also, Japanese Patent Application Laid-Open No. 2008-234315 discloses an image encoding method that efficiently performs tone compression on the basis of the response characteristics of human retina.
As shown in Report ITU-R BT.2246-1 (08/2012)/the present state of ultra high definition television, human visual characteristics are such that recognizable JND (Just Noticeable Difference) differs depending upon perceived luminance. However, in the tone compression shown in Japanese Patent Application Laid-Open No. 2008-234315, the fact that human visual characteristics differ depending upon perceived luminance is not taken into consideration. Therefore, encoding with high efficiency cannot be achieved when the display luminance of an output device is taken into consideration.